1. Field of the Invention
The invention relates to a fuel supply control method applied to an exhaust gas control apparatus for an internal combustion engine, which supplies fuel to a portion upstream of exhaust gas control means in order to control a temperature of the exhaust gas control means, for example, a NOx storage reduction catalyst to a target temperature. The invention also relates to an exhaust gas control apparatus to which the method is applied.
2. Description of the Related Art
In a NOx storage reduction catalyst used as exhaust gas control means for a lean-burn internal combustion engine (e.g., a diesel engine), a catalytic function thereof is reduced due to accumulation of sulfur oxides contained in exhaust gas. Therefore, when the NOx storage reduction catalyst is used, a recovery process, that is, so-called S recovery, needs to be periodically performed in order to decompose and remove the sulfur oxides accumulated in the catalyst thereby recovering the catalytic function. In the S recovery, a temperature of the catalyst (hereinafter, referred to as a “catalyst temperature” where appropriate) is increased to a target temperature (e.g., a temperature equal to or higher than 600° C.) that is higher than the upper limit of a temperature rage in a normal operating state, and an air-fuel ratio in a portion near the catalyst is maintained at the stoichiometric air-fuel ratio or a rich air-fuel ratio. The catalyst temperature is increased, for example, by adding fuel, as a reducing agent, to the exhaust gas. However, if a certain amount of fuel, which is required to control the catalyst temperature to the target temperature, is continuously supplied, a reductive reaction may occur continuously and therefore the catalyst temperature may increase excessively. In order to address this problem, for example, Japanese Patent Application Publication No. JP(A) 2003-166415 discloses an exhaust gas control apparatus which proceeds with the S recovery while suppressing overheating of the catalyst by alternately repeating a fuel supply mode and a fuel supply stopped mode instead of continuously supplying the amount of fuel that is required for the S recovery. Also, Japanese Patent Application Publication No. JP(A) 11-148399 and Japanese Patent Application Publication No. JP(A) 2001-82137 disclose technologies related to the invention.
In the exhaust gas control apparatus disclosed in each of the above-mentioned documents, a basic cycle of the fuel supply control is configured such that, after the entire amount of fuel that needs to be supplied during each cycle has been supplied, fuel supply is not performed during a period set based on the amount of supplied fuel. With such a configuration, the catalyst temperature fluctuates so as to be the lowest at each of the starting point and the ending point of each cycle, and so as to be the highest in the middle of the cycle. In this case, if the temperature is controlled such that the average temperature in each cycle becomes substantially equal to the target catalyst temperature, overheating of the catalyst and an insufficient increase in the catalyst temperature can be suppressed. However, as the length of the cycle becomes longer, the fluctuation range of the catalyst temperature in each cycle is increased. Accordingly, the catalyst temperature at each of the starting point and the ending point of the cycle also fluctuates based on the length of the cycle. Accordingly, when the cycles whose lengths are different from each other are alternately performed, the catalyst temperature at the starting point of the cycle performed later may fluctuate due to the effect of the catalyst temperature at the ending point of the cycle performed earlier. As a result, the catalyst temperature in the cycle performed later may deviate from the target temperature, and therefore overheating of the catalyst or an insufficient increase in the catalyst temperature may occur.